1. Field of the Invention
The present invention relates to a band pass filter which is useful in a communication apparatus.
2. Description of the Related Art
A communication apparatus which performs information communications by radio or wire is configured by various high-frequency components such as an amplifier, a mixer, and a filter. Among such components, a band pass filter has a function of allowing only a signal of a specific frequency band to pass through the filter. Some of such band pass filters are configured by arranging a plurality of resonators.
In a planar circuit configured by a microstrip line, a strip line, or the like, a coupling between resonators constituting a filter is usually defined only by positional relationships between the resonators, and realized without using a coupling element in addition to the resonators. This coupling method is suitable for a filter configured only by a coupling between adjacent resonators, such as a usual Chebyshev function type filter. In the case where a filter circuit having a cross coupling for a steepening of the skirt characteristic due to an attenuation pole or a flattening of the group delay time is to be realized, the coupling method has a problem that undesired couplings are easily generated in addition to a desired coupling between resonators.
On the other hand, the following documents (1) to (5) cited below disclose a method in which the cross coupling for a steepening of the skirt characteristic is realized by addition of a coupling line. In the coupling method, the ends of the coupling line are placed at positions where are close to two resonators and separated by a certain distance therefrom, whereby a coupling between the resonators is realized. In the following document (6), the electric length of a coupling line is variously changed to realize the flattening of the group delay time or the steepening of the skirt characteristic due to an attenuation pole. In the following documents (1) to (3), a quarter-wavelength coupling line is used. However, the techniques disclosed in the documents have a problem that parasitic couplings are easily generated between the ends of the coupling line and the resonators, and the resonance frequencies of the resonators are effectively deviated. In order to attain a strong coupling, the distances between the coupling line and the resonators must be very short. This causes another problem that a stable coupling cannot be obtained.
(1) JP-A-11-17405, (2) JP-A-2001-313502, and (3) JP-A-2002-76703 are referred to as related art.
Further, (4) IEEE Microwave Theory and Techniques Symposium Digest (1999), p. 1,547, (5) IEEE Microwave Theory and Techniques Symposium Digest (2000), p. 661, (6) IEEE Transactions on Microwave Theory and Techniques, No. 48 (2000), p. 1,240, (7) IEEE Microwave Theory and Techniques Symposium Digest (2002), p. 1,963, (8) IEEE Transactions on Microwave Theory and Techniques, No. 50 (2002), p. 2,924, and (9) IEEE Microwave Theory and Techniques Symposium Digest (2000), p. 319 are also referred to as related art.
As described above, in a coupling between resonators using a coupling line in a filter circuit, it is very difficult to prevent the resonance frequencies of the resonators from being deviated. Furthermore, it is impossible to stably realize a strong coupling.